Improve module mode:

- Don't hide module function inside `luaopen_%` function.
- Raise Lua exception instead of panic if module function returns error.

src/lua.rs

@@ -536,7 +536,15 @@ impl Lua {
         F: 'static + MaybeSend + Fn(&'callback Lua) -> Result<R>,
     {
         let cb = self.create_callback(Box::new(move |lua, _| func(lua)?.to_lua_multi(lua)))?;
-        unsafe { self.push_value(cb.call(())?).map(|_| 1) }
+        match cb.call(()) {
+            Ok(res) => unsafe { self.push_value(res)? },
+            Err(err) => unsafe {
+                self.push_value(Value::Error(err))?;
+                // This longjmp is undesired, but we cannot wrap it to a C
+                ffi::lua_error(self.state)
+            },
+        }
+        Ok(1)
     }
 
     /// Sets a 'hook' function that will periodically be called as Lua code executes.